The Fischer-Tropsch synthesis (FTS) process converts synthesis gas or syngas, a mixture of carbon monoxide and hydrogen, into liquid and gaseous hydrocarbon fuels. The syngas is produced by the gasification of coal, biomass, and other solid hydrocarbons in oxygen and steam at high temperatures and pressures. Typically, coal constitutes 75-100% of the gasification feedstock. The primary products of FTS are normally clean, high quality transportation fuels, including gasoline, jet fuel, and diesel fuel. The synthetic fuels resulting from the FTS process advantageously increase energy diversity. They also burn cleanly and thus hold the promise of improved environmental performance.
Currently there is greatly renewed interest in large scale development of FTS plants to convert coal, biomass, and other hydrocarbon feed stocks into liquid fuels. While state of the art FTS processes produce a very clean fuel, they also produce significant emissions of carbon dioxide, a greenhouse gas. This is because coal-derived syngas typically only has hydrogen to carbon monoxide (H2/CO) molar ratios in the range of approximately 0.6 to 1.1, dependent on the method of gasification. In order to produce liquid fuels by FTS, the H2/CO ratios of the syngas must be raised to values of 2.0 or higher.
State of the Art FTS technology relies on the water-gas shift (WGS) reaction,CO+H2O→CO2+H2,to raise the H2/CO ratio of the syngas to the required, values of 2.0 or higher. This reaction, unfortunately, produces one CO2 molecule for each H2 molecule it adds to the syngas. Unless the CO2 produced by the WGS reaction during the FTS process is captured and stored, for example, underground, state of the art FTS processes emit this CO2 to the atmosphere, thereby increasing the greenhouse effect. Current technology is focused on preventing this undesirable result by capture and storage of the CO2. Systems for the capture and storage of CO2, including proposed underground storage systems, are, unfortunately, quite expensive, largely untested, and add significant cost and complexity to the production of liquid transportation fuels by FTS.
The present invention relates to a modified and improved FTS process wherein the H2 required to increase the H2/CO ratio of the syngas to 2.0 or higher is produced by catalytic dehydrogenation (CDH) of the C1-C4 hydrocarbon products of FTS (CnH2n+2 (n=1 to 4) and CnH2n (n=2 to 4)). Conveniently, these hydrocarbons are gases at ambient conditions of temperature and pressure. CDH converts the C1-C4 products into H2 and multi-walled carbon nanotubes (MWCNT), a valuable by-product, with no production of CO2. Thus, the present invention represents a significant advance in the art allowing for a more environmentally friendly manufacture of liquid transportation fuels from coal and other solid hydrocarbons as an alternative fuel supply, together with a valuable by-product.